Apparatus and method for monitoring an airway device such as an endotracheal tube

ABSTRACT

An airway device ( 14 ), that is used to maintain a clear airway in a patient, e.g. for artificial ventilation during surgery, comprises an inflatable cuff ( 26 ), which is inflated when in position in a patient&#39;s airway ( 24 ). The inflated cuff ( 26 ) provides a seal to maintain the device ( 14 ) in position in a patient&#39;s airway ( 24 ), and to prevent leakage of infected oropharangeal secretions into the patient&#39;s lungs. A method and apparatus ( 10 ) for monitoring: leaks in the pressure system of the device ( 14 ) that includes the cuff ( 26 ); blockage in the pressure system, and/or malpositioning of the airway device ( 14 ) during use.

RELATED APPLICATIONS

This application is a divisional of, and claims priority under 35 U.S.C.§120 to U.S. patent application Ser. No. 13/621,983, which is adivisional application of, and claims priority under 35 U.S.C. §120 to,U.S. Patent Application Ser. No. 12/678,368.

TECHNICAL FIELD

The present invention relates to the monitoring of an airway device,such as an endotracheal tube or other pharyngeal/laryngeal device formaintaining an open airway in an animal or human patient.

BACKGROUND

An endotracheal tube, or similar airway device, is used to maintain aclear airway in a patient, e.g. for artificial ventilation duringsurgery. Typically, the device includes an inflatable cuff, which isinflated when in position in a patient's airway. The inflated cuff formsa seal, between the airway device and the tissues surrounding theairway, to thereby maintain the device in position in a patient'sairway, and to prevent leakage of infected oropharyngeal secretions intothe patient's lungs.

WO-A-99/33508 describes an apparatus that includes a monitoring systemfor monitoring pressure in the cuff of a laryngeal mask airway device,and for maintaining the inflated cuff at a preset pressure, withinnarrow tolerances. In particular, the apparatus periodically measuresthe pressure within a closed system incorporating the inflated cuff andcompares the measured pressure with a preset pressure, to determine adifference and its polarity. The apparatus then controls the pressurewithin the closed system to reduce the difference to zero. Thedisclosure of WO-A-99/33508 is incorporated herein, by reference.

SUMMARY

The present invention seeks to provide improvements over the existingapparatus described in WO-A-99/33508.

According to a first aspect, the present invention provides a method formonitoring a pressure of an inflatable seal of an airway device, themethod comprising: periodically receiving a pressure value indicative ofthe pressure of the inflatable seal; comparing the received pressurevalue with a preselected, desired pressure value, and, if a differencebetween the received pressure value and the preselected pressure valueis greater than a predefined tolerance, changing the pressure of theinflatable seal, to reduce the difference, characterised in that thestep of changing the pressure is carried out at a rate dependent upon adifference between the received pressure value and the desired pressurevalue.

In prior art techniques, it is typical to respond quickly to correctpressure deviations from a set point. However, cyclic pressure changesin the inflatable seal occur as a result of intra-thoracic andintra-tracheal pressure from the patient's airway during the breathingcycle. In certain circumstances, responding instantaneously to correctpressure deviations resulting from normal cyclic changes may reduce theinflation pressure to below a minimum required to create a seal with theairway. This may lead to leakage of secretions past the seal into thepatient's lungs, and increase the risk that the airway device may bedislodged.

Accordingly, the present invention minimises this problem, bycontrolling the rate at which the pressure is changed in response to adetected deviation or change in pressure from the desired pressure.Thus, instead of making an instantaneous change in pressure to correctfor the deviation, the pressure is adjusted at a rate dependent upon thedeviation of the measured pressure value from the desired pressurevalue. The rate of adjustment may be further limited by a thresholdvalue. By controlling the rate of pressure adjustment, the time taken tocorrect for the deviation from the desired value is delayed. Typically,the rate of adjustment produces a delay for a period that correspondsto, or is greater than, a normal time period for inspiration orexpiration, which represents the greatest deviation in pressureassociated with the breathing cycle.

In one embodiment, the rate of change of pressure is dependent on thesquare of the difference between the measured pressure value and thedesired value.

For example, in an embodiment in which a piston is utilised to changethe volume, and hence the pressure, in a closed volume system, thepiston is moved at a rate N that is defined by the equation:

$N = \frac{( {( {P_{1} - S} )^{2}*S} )}{(R)}$

Where:

-   -   P₁ is the actual measured pressure    -   S is the set point or desired pressure    -   * is the multiplier operation    -   R is a regulation constant

In one embodiment, the rate N is a speed that corresponds to the numberof steps per second moved by a stepper motor driving the piston.

The rate N may be subject to a predetermined maximum threshold speed.Thus, if the calculated rate N is greater that the threshold, the pistonis controlled to move at the threshold rate.

Typically, if the measured pressure is higher than the set-point/desiredpressure, the motor drives the piston backward at rate N in order toreduce pressure, and if the measured pressure is lower than set-pointmotor drives the piston forward at rate N in order to increase pressure.

In one embodiment, the measured pressure value is received at periodictime intervals, for example time intervals in the range of 0.1 to 2.5seconds, and the rate of change of volume, and thus pressure,recalculated and adjusted. In one embodiment, the rate is recalculatedevery 0.5 seconds.

According to the second aspect, the present invention provides a methodfor detecting a change in position of an airway device with aninflatable seal, the method comprising: comparing a received pressurevalue indicative of a pressure of the inflatable seal with a predefinedpressure value to determine a difference, and, if the difference isgreater than a predetermined amount for a predetermined time period,indicating a detected change in position of the airway device.

A change in position of an airway device, such as an endotracheal tube,may arise during medical or surgical procedures and may be hidden (i.e.not detectable by those monitoring the patient). Such changes may leadto extubation, whereby the endotracheal tube cuff is unintentionallywithdrawn into the patient's larynx or airway above. However, since thelaryngeal volume is normally considerably greater than that of thetrachea, it is possible to detect such hidden extubation, in accordancewith the present invention, by monitoring for a change, in particular asubstantial drop, in pressure.

In one embodiment, the predefined pressure value is a previous pressurevalue that is within predefined tolerances of a preselected, desiredpressure value. In another embodiment, the predefined pressure value isa preselected, desired pressure value.

In one embodiment, the predetermined amount is a proportion of apredefined pressure value. In one embodiment, the predetermined amountis 20% of the predefined pressure value, so that a change in position isdetected if the received pressure value is below 80% of the predefinedpressure value for a predetermined time period.

The predetermined time is typically in the region of 15 seconds to 2minutes, and in one embodiment is 60 seconds.

In one embodiment, the method comprises: receiving a first pressurevalue indicative of a pressure of the inflatable seal, the firstpressure value within predefined tolerances of a preselected, desiredpressure value; thereafter, periodically receiving further pressurevalues at predetermined time intervals; comparing each received furtherpressure value with the first pressure value to determine a difference;if the difference is greater than a predetermined amount, starting atimer for a predetermined time period, and, if the further, periodicallyreceived further pressure values are not within the predefinedtolerances of the preselected pressure value before the timer hasexpired, indicating a detected change in position of the airway device.

It will be appreciated that in other embodiments, the difference betweenthe received pressure value and the predefined pressure value need notbe calculated. Rather, the received pressure value may be compared witha minimum value corresponding to predetermined proportion of thepredefined pressure value (e.g. 80% of the predefined pressure value),and if the received value is less that the minimum value, the timerstarted.

According to a third aspect, the present invention provides a method fordetecting a leak of fluid from an inflatable airway device, the methodcomprising: receiving a pressure value for the inflatable airway device;calculating a statistical average of values calculated using thereceived pressure value and a predetermined number of previouslyreceived pressure values, and, if the statistical average exceeds athreshold average, indicating a leak of fluid from the airway device.

In one embodiment, the values calculated using the received andpreviously received pressure values are values that represent a rate ofchange in volume or pressure of fluid in the inflatable airway device.Preferably, the rate of change in volume or pressure corresponds to achange to increase the pressure in the inflatable airway device.

A leak of fluid used to inflate and pressurise a seal of an airwaydevice can adversely affect the proper functioning of the airway device.It is therefore important to monitor for and detect leaks of inflationfluid, to enable these to be manually corrected, wherever possible. Inaccordance with the present invention, this can be achieved bymonitoring the pressure of the inflatable seal.

During use of an airway device, it is possible that a leak may bepresent in the airway device. Such a leak may be caused by perforationduring surgery, or may be a slow leak from connections in the pressuresystem. According to the present invention, if a leak occurs whilst theairway device in use, it can be detected by calculating a statisticalaverage of a number of values, that are each dependent on acorresponding number of previously received pressure values, andcomparing it with a threshold statistical average. For example, thevalue may be a calculated rate of change of volume or pressure, asdetermined in accordance with the first aspect of the present invention.If the average is above a threshold, this indicates significant changeshave been necessary over an immediately preceding time interval, toincrease pressure to the desired pressure, thus signifying a possibleleak of fluid.

According to a fourth aspect, the present invention provides a methodfor detecting a leak of fluid from an inflatable airway device, themethod comprising: periodically receiving a pressure value for theinflatable airway device; comparing the received pressure value with apredetermined, minimum pressure value, and if the received pressurevalue is less than the predetermined minimum pressure value, indicatinga leak of fluid from the airway device.

In one embodiment, the predefined minimum pressure value is apredetermined proportion of a preselected, desired pressure value, forexample 50% of the preselected pressure value.

When an airway device is first introduced into a patient, and inflated,pressure adjustments are made until a desired pressure value isachieved. Thereafter, the pressure value is periodically monitored todetect a sudden drop in pressure. In accordance with the presentinvention, if the measured pressure value is less than a predeterminedproportion of the preselected pressure value, for example 50%, thissignifies a sudden loss of fluid from the pressure system, and thus aleak is detected.

According to a fifth aspect, the present invention provides a method fordetecting a blockage in a pressure system of an inflatable airwaydevice, the method comprising: receiving a pressure value for theinflatable airway device and calculating a current activity, and, if thecurrent activity is below a threshold for activity for a predeterminedtime period, indicating a blockage in the pressure system of the airwaydevice.

In one embodiment, activity is calculated by comparing the receivedpressure value with a desired or average pressure to calculate adifference. Activity is then determined by as a statistical average ofthe difference and a predetermined number of differences calculatedusing previously received pressure values.

Solid material can be introduced into a closed volume pressure system ofan airway device in certain circumstances. This can lead to a blockage,adversely affecting the proper functioning of the pressure monitoringsystem. When a blockage occurs, the pressure becomes substantiallystatic, and normal cyclic changes of pressure, associated withventilation, are not detected. The present invention detects a blockagein the pressure system of an airway device if the average value ofdeviations in pressure from a desired value, over a predetermined timeperiod, is below a threshold. The threshold is typically less that anaverage of deviations over the predetermined time period that would beexpected to arise as a result of the breathing cycle.

In accordance with a sixth aspect, the present invention provides acomputer readable medium comprising program instructions which, whenexecuted, perform the method of any one or more of the first, second,third, fourth and fifth aspects of the present invention.

In accordance with the seventh aspect, the present invention provides anapparatus comprising processor means, configured for carrying out themethod of any one or more of the first, second, third, fourth and fifthaspects of the present invention.

As the skilled person with appreciate, each of the methods of the first,second, third, fourth and fifth aspects of the present invention can beutilised in combination, and is typically implemented in the form ofsoftware, executed on a computer processor. It will be appreciated thatthe methods of the present invention may equally be implemented in theform of hardware.

Other desired and optional features and advantages of the presentinvention will be apparent from the following description andaccompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a schematic, perspective view of an apparatus in accordancewith an embodiment of the present invention, coupled to an airway devicein position in a patient;

FIG. 2 is a schematic block diagram illustrating the apparatus of FIG.1;

FIG. 3 is a block diagram illustrating the control and monitoringapparatus of FIG. 1, which implements the method of embodiments of thepresent invention;

FIG. 4 is a flow diagram illustrating the method steps performed forcontrolling pressure in an airway device, in accordance with a firstembodiment of the present invention;

FIG. 5A and FIG. 5B are a flow diagrams, illustrating the method stepsperformed for detecting a positional change of an airway device, inaccordance with a second embodiment of the present invention;

FIG. 6A and FIG. 6B are flow diagrams, illustrating the method stepsperformed for detecting a leak in the pressure system of an airwaydevice, in accordance with a third embodiment of the present invention;

FIG. 7 is a flow diagram, illustrating the method steps performed fordetecting a leak in the pressure system of an airway device, inaccordance with a fourth embodiment of the present invention, and

FIG. 8 is a flow diagram, illustrating the method steps performed fordetecting a blockage in the pressure system of the airway device, inaccordance with a fifth embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description of embodiments of the present invention,the described monitoring and control apparatus includes a closed volumepressure system which includes an inflatable seal of an airway device.In the described embodiments, an endotracheal tube is described, by wayof example of a suitable airway device. The skilled person willappreciate that at least some of the embodiments may be implemented inconjunction with other forms of airway device, such as a laryngeal maskairway device. In addition, whilst the use of a closed volume system isadvantageous, at least some of the embodiments may be used inconjunction with other systems for monitoring and controlling thepressure of an inflatable seal of an airway device.

FIG. 1 shows a pressure control and monitoring apparatus 10, accordingto an embodiment of the present invention, connected to a flexibleinflation/deflation air supply line 12 of an airway device 14 positionedfor use in maintaining a clear airway in a patient 16.

The airway device 14 comprises an airway tube 18 with a proximal end 20for connection to an external ventilating or anaesthetizing supply tothe patient's lungs and a distal end 22 for insertion into the trachea24 of a human or animal patient. As shown in FIG. 1, the distal end 22of tube 18 is peripherally surrounded by an inflatable/deflatable ringor cuff 26 of resiliently flexible material. The cuff 26, which isformed from a flexible, resiliently deformable material, is shown ininflated condition in FIG. 1 and forms a seal between the airway device14 and the surrounding body structure of the patient 16, i.e. the wallof the trachea 24.

The structure of airway device 14 is conventional, and well known in theart. Examples of a suitable endotracheal tube include the Portex SoftSeal tracheal tube and the LoTrach tracheostomy tube.

Typically, the cuff 26 of the airway device 14 is manually inflated,once positioned in a patient, as shown in FIG. 1. Thereafter, thepressure monitoring and control system 10 can be utilized by connectingthe airway device 14 to inflation/deflation air supply line 12 ofapparatus 10.

FIG. 2 shows schematically components of the pressure monitoring andcontrol system 10, which comprises an air-control port 30 adapted fordetachable connection to the inflation/deflation air supply line 12. Anelongate flexible connection or extension line 12′ (FIG. 1) isdetachably connectable to control port 30 at one end, and, at the otherend, is connectable to the connector end of a check-valve 12″. In thisway, air lines 12, 12′ provide a continuously open passage of a pressuresystem in communication with inflatable cuff 26 of the airway device 14and an air displacement mechanism within the housing of apparatus 10, asdescribed below.

Air-displacement mechanism comprises a syringe in the form of a body 32of low-friction material such as PTFE with a cylindrical bore 34 havingan open (or tail) end for coaction with a piston 36. Body 32 is fixed toa frame and extends longitudinally to a closed (or head) end having aport connection 40 to a direct line to air control port 30. In thisdirect line, a first normally closed solenoid valve V1 must be actuatedto open condition if inflation air is to pass in either directionbetween cylinder 34 and the inflatable/deflatable cuff 26 of the airwaydevice 14.

Piston 36 is rigidly mounted to a support and guide means (not shown) toenable the piston 36 to be driven longitudinally through the cylindricalbore 34, with precise alignment with the central axis of the bore.Piston 36 is driven by step motor 44, which has a precise directionalcontrol via variance of the relative excitation of each of four inputterminals under the control of motor controller and driver means 48, asshown in FIG. 3. Motor controller and driver means 48 operates inresponse to commands from a processor 50 (not shown) to change thepressure in the cuff 26, by altering the volume of the closed volumesystem formed between syringe body 32 and cuff 26 of the airway device14. In particular, adjustments for cuff pressure deviation are made byincrements of displacement of air in the closed volume system, eachincrement (corresponding to a step of motor 44) moving, for example,approximately 0.0005 ml. This enables pressure adjustments with a highdegree of accuracy.

The apparatus of the present invention measures the instantaneouspressure in the closed volume system. In particular, apparatus 10includes first and second pressure sensors PS1, PS2, which are connectedto redundantly monitor air pressure in the line between cylinder-outletport 40 and the normally closed first solenoid valve V1. Thus, except inthe case of error, first and second sensors PS1, PS2 should produce thesame pressure reading A second normally closed solenoid valve V2 isshown connected to the air line between cylinder port 40 and the firstsolenoid valve V1. When actuated to open condition, valve V2 establishesa path from its open-air end to the air line from cylinder port 40 tothe first solenoid valve, so that, with valve V1 in its closedunactuated condition and with valve V2 actuated to its open condition, aright to-left (backward) displacement of piston 36 in cylindrical bore34 will induce an inflow of fresh (ambient) air into the closed volumepressure system. Similarly, with the two valves V1 and V2 in the samecondition (of V2 actuated and of V1 in its normally closed condition), aleft to-right (forward) displacement of piston 36 in bore 34 willdischarge excess air or gas from the system.

Also, and analogously, with valve V2 in its normally closed conditionand with valve V1 actuated to its open condition, a right-to-left(backward) displacement of piston 36 will draw inflation air from (andthus deflate) cuff 26 of the airway device 14, and for the sameconditions, of valve V2 unactuated and of valve V1 actuated, aleft-to-right (forward) displacement of piston 36 will supply inflationair to inflatable cuff 26 of the airway device, thereby increasingpressure.

Control signals necessary for actuation of valves V1 and V2 are providedby program-sequencing signals from the processor 50 of the apparatus 10(as shown in FIG. 3 described below) in accordance with conventionaltechniques.

FIG. 3 shows logic components of the apparatus 10 for monitoring andcontrolling the inflation pressure of the cuff 26 of an airway device14, for example, as shown in FIG. 1. The apparatus 10 comprises aprocessor 50 that receives input signals from the first and secondpressure sensors PS1, PS2 and provides control signals to the step motorcontroller and driver 48, and the valves V1 and V2, and outputs signalsto a user interface 52 to provide information such as the selected andmeasured pressure values, and to provide alarm indications, inaccordance with one or more methods of embodiments of the presentinvention.

As the skilled person will appreciate, processor 50 may comprise aprogrammable microprocessor, microcontroller or the like, that executesprogram instructions to perform methods in accordance with embodimentsof the present invention. Alternatively, parts of the functionality ofprocessor 50 may be conveniently implemented in hardware, for example asdescribed in WO-A-99/33508.

Apparatus 10 operates in a similar manner to the apparatus described inWO-A-99/33508. In particular, the System Start Up, Failsafe and NormalSystem Control operations are analogous to those described inWO-A-99/33508, and so will not be described in detail herein.

As described above, apparatus 10 monitors and regulates the inflationpressure of the airway device cuff 26. A patient is typicallymechanically ventilated, i.e., the patient's ventilation is the resultof positive pressure being exerted through airway tube 18, also referredto as intermittent positive-pressure ventilation or “IPPV”. As a result,apparatus 10 measures oscillations in pressure about a midline pressurevalue. These oscillations correspond to the respiratory cycle, which isabout twelve cycles per minute for a typical anaesthetized adultpatient. Thus, respiratory flow through airway tube 14, whetherspontaneous or by IPPV, will have the effect of causing the cuff 26 toundergo regular cycles of compression/expansion. This results in cyclicchanges in pressure, due to the change in volume of the resilientlydeformable cuff 26 caused by this compression/expansion.

Whilst the known system of WO-A-99/33508 does not perform pressureadjustments when a pressure difference is detected within a toleranceband that is centred at a setpoint (corresponding to the midline), toallow for the above described oscillations, the system respondsimmediately when a pressure difference outside these tolerances isdetected. In particular, the system controls piston 36 so that it isimmediately displaced an appropriate distance to correct for themeasured deviation from the setpoint. However, this may cause thepressure in the cuff 26 to overshoot or undershoot the tolerance bandaround the setpoint due to changes caused by the respiratory cycle. As aresult, the cuff pressure may drop below the minimum pressure requiredto create a seal, risking leakage of secretions and possible movement ofthe airway device, or may exceed a maximum pressure, risking possibleinjury to the patient.

FIG. 4 is a flow diagram illustrating the steps performed by processor50 implementing a method for adjusting the pressure of the closed volumesystem of the above-described device in accordance with a firstembodiment of the present invention. The method corrects for deviationsfrom a desired pressure, outside a predetermined tolerance band, whilstensuring that the inflation pressure does not drop below a minimumrequired to create a seal with the airway. In particular, the methodcontrols the rate at which pressure is changed in response to detectedpressure deviations, to allow for deviations resulting from normalpressure changes associated with the breathing cycle of the patient.

The method starts after the pressure P₁ of the inflatable cuff 26 of theairway device 14 has been established within a predefined tolerance bandcentred at a preselected set point pressure value, in accordance withthe conventional System Start-Up procedure. As the skilled person willappreciate, the setpoint is chosen by the operator from a number ofpossible midline pressure values which are typically used for differentpatients such as children, male and female adults. In this embodiment,cuff pressures values of 20, 30, 50 and 50 mmHg may be selected by theoperator, although the apparatus may provide for other suitable pressurevalues to be selected.

Accordingly, at step 405, the method receives a measured pressure valuefrom one of the first and second pressure sensors PS1, PS2 of apparatus10 corresponding to a pressure P₁ of the inflatable cuff. At step 410,the method compares the measured pressure value P₁ with the selectedsetpoint value S, to determine a difference (P₁S), and considers whetherthe difference is within the predefined tolerance band (e.g. S+/−0.5mmH2O (mmHg)). If, on the one hand, step 410 determines that themeasured pressure value P₁ is within the predefined tolerance band, themethod returns to step 405 and waits for the next sampled pressurevalue. On the other hand, if step 410 determines that the measuredpressure value P₁ is not within that the predefined tolerance band aboutthe setpoint value, the method continues to step 415 by determining arate of change of the pressure, to correct for the difference. Inparticular, in the illustrated embodiment, step 415 calculates a valuefor the speed N for the piston 36 to be driven by stepper motor 44,where N is the number of steps of the stepper motor per second, usingthe equation:

$\begin{matrix}{N = \frac{( {( {P_{1} - S} )^{2}*S} )}{(R)}} & (1)\end{matrix}$

Where:

-   -   N is the number of steps    -   P₁ is the actual measured pressure    -   S is the set point or desired pressure    -   * is the multiplier operation    -   R is a predetermined regulation constant

A regulation constant is used to smooth the regulation of pressure andprevent oscillation of the system.

In one embodiment, the value of speed N is limited by a predefined,maximum threshold value T for the number of steps N per second for thestepper motor 44. In one embodiment, T is 2000 steps per second.Limiting the speed in this way ensures that the reaction time is alwaysdelayed, thereby ensuring that the pressure does not overshoot orundershoot the tolerance band. Thus, if step 415 calculates a value Nthat exceeds the threshold value, the threshold value T is instead usedfor the motor speed N.

Next, at step 420, the method determines if the sampled pressure P₁ isgreater than the setpoint value, in order to determine the direction ofmovement of the piston. If step 420 determines that the pressure P₁ isgreater than the setpoint, the method continues with step 425, bysending a signal to stepper motor controller and driver 48 to controlthe stepper motor 44 to drive the piston 36 at speed N backwards in thecylindrical bore 34 to increase the volume of, and thus reduce thepressure in, the closed volume pressure system, and thus reduce pressurein the inflatable cuff 26. Alternatively, if step 420 determines thatthe pressure P₁ is less than the setpoint, the method continues withstep 430, by sending a signal to stepper motor controller and driver 48to control the stepper motor 44 to drive the piston at speed N forwardsin the cylindrical bore 34 to reduce the volume of, and thus increasethe pressure in, the closed volume pressure system, and thus increasepressure in the inflatable cuff 26.

Following either step 425 or 430, the method returns to step 405 andwaits for the next sampled pressure value. Typically, pressure valuesfor P₁ are periodically received at regular time periods, for exampleevery 0.5 seconds. It will be appreciated that in other embodiments,other values for the time period between samples may be used, andconsequent changes in the equation for calculating the speed of thepiston, may be used. However, sampling the pressure every 0.5 seconds incombination with the calculation of the rate of change ofvolume/pressure using the above equation (1) has been found to producegood results, since the speed N is recalculated after only half thepreviously calculated number of steps to correct the pressure have beencompleted.

Thus, the method of this embodiment of the present inventionperiodically samples the pressure of the system and responds to changeswith a reaction speed that is dependent upon the difference between themeasured pressure and the desired pressure (i.e. setpoint), subject to apredefined, maximum threshold value. Thus, the regulation of thepressure, in the closed volume system, is continually varied by changingthe piston speed, which effects the change in pressure. The methoddrives the piston 36 at high speed when the difference between actualand set-point pressure is high and will slow down the piston speed oncethe measured pressure gets close to the set-point pressure. Thus, asmother regulation of pressure is achieved by controlling the rate ofchange in pressure in response to detected pressure deviations. Bymaking gradual changes to the pressure, to correct for deviation fromthe desired pressure, the problems associated with making instantaneouschanges may be obviated.

FIG. 5A is a flow diagram illustrating the steps performed by processor50 implementing a method for detecting a change in position of aninflated airway device 14 in accordance with another embodiment of thepresent invention. As explained above, a change in position of an airwaydevice, such as an endotracheal tube, may arise during medical orsurgical procedures. Such changes may lead to extubation, whereby theendotracheal tube cuff is unintentionally withdrawn into the patient'slarynx or upper airway. The laryngeal volume is normally considerablygreater than that of the trachea, and it is therefore possible to detectsuch hidden extubation by monitoring for a substantial drop in pressure,typically a drop in pressure of 20% or more. The method starts afterfirst establishing a pressure P₁ of the inflatable cuff of the airwaydevice 14 within a predefined tolerance band, centred about apreselected set point pressure value, in accordance with a conventionalSystem Start-Up procedure. As the skilled person will appreciate, thesetpoint is chosen from a number of possible midline pressure valueswhich are typically used for different patients such as children, maleand female adults. In this embodiment, cuff pressures values of 20, 30,40 and 50 mmHg may be selected by the operator, although the apparatusmay provide for other suitable pressure values to be selected.

Accordingly, at step 505, the method receives a measured pressure valuefrom one of the first and second pressure sensors PS1, PS2 of apparatus10 indicating the pressure P₁ of the inflatable cuff. At step 510, themethod compares the measured pressure value P₁ with the selectedsetpoint value, to determine a difference, and considers whether thedifference is within the predefined tolerance band. If step 510determines that the measured pressure value P₁ is not within thepredefined tolerance, the method returns to step 505.

Alternatively, if step 510 determines that the measured pressure valueP₁ is within that the predefined tolerance band, the method continues tothe second stage at step 515.

At step 515, the method receives a further measured pressure value P₂from one of the first and second pressure sensors of apparatus 10. Step520, compares the further measured pressure value P₂ with the previousmeasured pressure value P₁ to determine a difference, and considerswhether the difference is greater than a predetermined amount, in thiscase 20% of the previous measured pressure value P₁. It will beappreciated that the predetermined amount might equally be a differentproportion of the previous measured value, for example in the range of15% to 30%, a corresponding proportion of the setpoint value, or afixed, predetermined pressure difference (e.g. 10 to 25 mmHg).

If step 520 determines that the difference is not greater than the 20%,the method returns to step 515. Alternatively, if step 520 determinesthat the difference is greater than 20% of the previous measuredpressure value P₁ (or alternatively the setpoint value or fixed pressuredifference), the method continues with step 525.

As the skilled person will appreciate, instead of comparing the measuredpressure value with the previous pressure value to determine adifference, and then comparing the difference with a threshold, it wouldbe equally possible for step 520 to compare the measured pressure valueP₂ with a predetermined minimum pressure value to achieve the sameresult. For example, the minimum pressure value could be 80% of theprevious of predetermined pressure value P₁. In that case, step 520would compare P₂ with 80% of P₁ and determine if P₂ is less than orequal to 80% of P₁, and if so proceed with step 525.

At step 525, the method starts a timer. In this embodiment, the timer isset for 60 seconds, but it will be appreciated that in the embodiments,alternative durations for the timer may be used, for example in therange 15 seconds to 2 minutes.

Whilst the timer, started in step 525, is running, the method continueswith step 530 by receiving a next measured pressure value P_(N). At step535, the method compares the next measured pressure value P_(N) with thesetpoint value to determine a difference, and considers whether thedifference is within the predefined tolerance band. If step 535determines that the measured pressure value P_(N) is within thepredefined tolerance band, centred about the setpoint value, the methodcontinues to step 540 which stops the timer. The method then returns tostep 515. Alternatively, if step 535 determines that the measuredpressure value P_(N) is not within the predefined tolerance band, themethod continues with step 545 by considering whether the timer hasexpired. If the timer has not expired, the method returns to step 530 byreceiving a next measured pressure value P_(N) (as mentioned above,typically, pressure values are sampled at periodic time intervals, forexample every 0.5 seconds). Alternatively, if the timer has expired, themethod continues to step 550 by determining a significant change inposition of the airway device, and activating a “Malposition Alarm”.

The step 550 of activating a Malposition Alarm may be to provide one ormore signals to sound an audible alarm and/or a visual alarm on userinterface 52 formed on the housing of the apparatus 10.

Following step 550, the method stops, but may be repeated once theairway device 14 has been repositioned and the apparatus 10 reset.

It will be appreciated that in some embodiments, the method of FIG. 5Amay be used in conjunction with a method that automatically activates analarm, if the measured pressure drops by a substantial amount, such as50% or more of the desired pressure value. Such a method is described inconnection with FIG. 7 below.

FIG. 5B is a flow diagram illustrating an algorithm implemented by themethod of FIG. 5A.

FIG. 6 is a flow diagram illustrating the steps performed by processor50 implementing a first method for detecting a leak of fluid, typicallyair, from a closed volume pressure system of an inflatable airwaydevice, in accordance with another embodiment of the present invention.

A leak of air or other fluid used to pressurise an inflatable seal of anairway device can adversely affect the proper functioning of thepressure monitoring and control system 10. It is therefore important tomonitor for and detect leaks, to enable these to be manually corrected,wherever possible. In accordance with the present invention, this can beachieved by monitoring the pressure of the inflatable seal.

During use of an airway device, it is possible that a leak may bepresent in the airway device. Such a leak may be caused by perforationduring surgery, or may be a slow leak from connections in the pressuresystem. According to the present invention, if a leak occurs whilst theairway device in use, it can be detected by calculating a statisticalaverage of a number of values, that are each dependent on acorresponding number of previously received pressure values, andcomparing it with a threshold statistical average. For example, thevalue may be a calculated rate of change of volume or pressure, asdetermined in accordance with the first aspect of the present invention.If the average is above a threshold, this indicates significant changeshave been necessary over an immediately preceding time interval, toincrease pressure to the desired pressure, thus signifying a possibleleak of fluid.

This is implemented in the method of FIG. 6, which is performed duringand/or after the conventional System Set-up procedure of the apparatus10. In particular, at step 605, the method receives a measured pressurevalue P_(N) from one of the first and second pressure sensors PS1, PS2of apparatus 10 corresponding to a pressure of the inflatable cuff 26.Step 610 the method compares the measured pressure value P_(N) with theselected setpoint value, to determine a difference, and considerswhether the difference is within a predefined tolerance band. If step610 determines that the measured pressure value P_(N) is within thepredefined tolerance band centred about the setpoint value, the methodsets a value N to zero at step 615. The method then returns to step 605,and receives the next sampled pressure value after a predefined samplingperiod. Alternatively, if step 610 determines that the measured pressurevalue P_(N) is not within the predefined tolerance band, the methodcontinues with step 615.

At step 620, the method determines a rate of change of volume, and thuspressure, to correct for the difference. In particular, in theillustrated embodiment, step 615 calculates a value for the speed forthe piston 36 to be driven by stepper motor 44 corresponding to thenumber of steps N of the stepper motor per second, e.g. by usingequation (1) above.

At step 625, the method determines whether the measured pressure valueP_(N) is greater that the setpoint or desired pressure value. If themeasured pressure value P_(N) is greater than the setpoint or desiredpressure value, then the setpoint pressure has been surpassed, and therecan be no leak, and in some embodiments, the method may then stop.However, in the present embodiment, if step 625 determines the measuredpressure value P_(N) is greater than the setpoint or desired pressurevalue, the method sets the value N as negative at step 628. Otherwise,if the measured pressure value P_(N) is less that the setpoint pressurevalue, the value for N is positive and the method continues with step630.

Step 630 calculates a statistical average for the rate of change ofvolume/pressure (e.g. calculated as a mean average) for a previouspredetermined number of samples, for example, the previous 20 samples(i.e. the value of N calculated for P_(N) and the value of N calculatedfor previous 19 samples pressure values). At step 635, the methodcompares the average calculated in step 630 with a predefined threshold.The threshold is a minimum average value that is indicative of aineffective operation of pressure regulation in response to pressurereductions arising during normal usage in a patient. In a preferredembodiment, the threshold is 400, which represents a normal averagevalue for N over 10 seconds of a breathing cycle (N including inflation(positive), deflation (negative) and static steps). The value of thethreshold may be adjustable by the operator, and may be set low, todetect minor leaks, or high to detect only more substantial leaks.

If step 635 determines that the average value is below the threshold,the method returns to step 605. Alternatively, if step 635 determinesthat the average value is above the threshold, the method continues withstep 640.

Step 640 indicates that there is a leak in the closed volume pressuresystem of the airway device, and thereby activates a “Leak Alarm”.

The step 640 of activating a Leak Alarm may be to provide one or moresignals to sound an audible alarm and/or a visual alarm on the userinterface 52 on the housing of the apparatus 10. These alarms signal tothe operator to check the connections in the closed volume pressuresystem, to try to remedy the loss of air.

Following step 640, the method stops. The method may be repeated oncethe apparatus 10 has been reset.

It will be appreciated that whilst the method of FIG. 6A determines anaverage for the calculated value N, and compares it with a threshold, inother embodiments other values may be used such as the average measuredpressure value. By using an average value, the method takes account ofnormal pressure fluctuations associated with the breathing orventilation cycle.

FIG. 6B is a flow diagram illustrating an algorithm implemented by themethod of FIG. 6A.

FIG. 7 is a flow diagram illustrating the steps performed by processor50 implementing another method for detecting a leak of fluid, typicallyair, from an inflatable airway device, in accordance with anotherembodiment of the present invention.

As explained previously, during use of the apparatus of the presentinvention, it is possible that a leak may be caused in the airwaydevice, for example due to perforation during surgery. According to thepresent invention, if a leak occurs whilst the airway device in use, itcan be detected by monitoring for a sudden drop in the pressure valuefor the inflatable airway device below a minimum pressure. Typically,the minimum pressure value is 50% of the desired or setpoint pressurefor the airway device.

This is implemented in the method of FIG. 7, which is performed afterthe Setup procedure of the apparatus 10, i.e. when the apparatus is innormal use.

At step 705, the method receives a measured pressure value P_(N) fromone of the first and second pressure sensors PS1, PS2 of apparatus 10corresponding to a pressure P_(N) of the inflatable cuff 26.

At step 710, the received measured pressure value P_(N) is compared witha predefined pressure value, in this embodiment corresponding to 50% ofthe selected setpoint value, to determine if P_(N) is less than 50% ofthe setpoint value S. If step 710 determines that the measured pressurevalue is more than 50% of the setpoint value, the method returns to step705. Alternatively, if step 710 determines that the measured pressurevalue is less than 50% of the setpoint value, the method continues tostep 715 by indicating a leak in the air inflation system of the airwaydevice, and thereby activating a “Leak Alarm”.

The step 715 of activating a Leak Alarm may be to provide one or moresignals to sound an audible alarm and/or a visual alarm on the userinterface 52 of the housing of the apparatus 10. This may be the same ora different alarm to that of step 635 of FIG. 6. This signals to theoperator to check the connections in the closed volume pressure system,to try to remedy the loss of air.

Following step 715, the method stops. The method may be repeated oncethe apparatus 10 has been reset.

FIG. 8 is a flow diagram illustrating the steps performed by processor50 implementing a method for detecting a blockage in the pressure systemof an inflatable airway device, in accordance with yet anotherembodiment of the present invention.

As discussed above, solid material can be introduced into a closedvolume pressure system of an airway device 14 attached to a pressuremonitoring and control apparatus 10 in certain circumstances. This canlead to a blockage, typically in the inflation line 12, adverselyaffecting the proper functioning of the monitoring and control apparatus10. When a blockage occurs, the pressure becomes substantially static,and normal cyclic changes of pressure, associated with ventilation, arenot detected. The present invention detects a blockage in the pressuresystem of an airway device if the average value of deviations inpressure from a desired value (herein termed “activity”), is below athreshold for a predetermined time period. The threshold is typicallyless that an average of pressure deviations over the predetermined timeperiod that would be expected to arise as a result of the breathingcycle. The method periodically calculates a value for activity,typically every 1.25 seconds, as follows. At step 805 an instantaneouspressure value P_(N) is received from one of the first and secondpressure sensors PS1, PS2 of apparatus 10 indicating the pressure of theinflatable cuff 26. At step 810 the difference between the measuredpressure value and a predefined (or precalculated) average pressurevalue is determined and at step 815 the difference is stored in acircular buffer, which stores the current and a predetermined number ofpreviously calculated pressure differences (in this case 10 values intotal). Step 820 calculates the Activity value as the mean average ofthe values stored in the circular buffer.

This mean average is an indicator of the activity which normally arisesfrom the breathing cycle. In particular, during normal operation, cyclicpositive and negative pressure fluctuations occur around the set-pointvalue due to either the patient spontaneously breathing or themechanical ventilation applied. During each cycle, the difference inpressure from the setpoint may vary from 0.5 up to 3-5 cm H₂O (1mmHg=1.395 mmH2O).

The method uses a timer to determine whether the activity levels arebelow a threshold for a significant time period indicative or ablockage. Typically the timer is triggered by a lower threshold foractivity (e.g. the lower threshold is 3) and runs for 3 minutes (180seconds) although other time periods between 1 to 5 minutes arepossible. It the timer expires without being reset, then a Blockagealarm is activated. However, if the activity rises above an upperthreshold (e.g. 8) before the timer expires, the timer is reset orstopped.

It will be appreciated that the Blockage alarm may be to provide one ormore signals to sound an audible alarm and/or a visual alarm on the userinterface 52 on the housing of the apparatus 10. This signals to theoperator to check for a blockage in the closed volume pressure system.

Referring again to FIG. 8, step 825 determines whether the timer hasexpired, and, if so, sets a Blockage alarm flag at step 830 to initiatea Blockage alarm. Following step 830, the method compares the currentactivity value with an upper threshold value at step 835 and determinesif the activity is greater than the upper threshold. The upper thresholdis a value for activity indicative of normal pressure fluctuationsassociated with the breathing cycle. If the activity is greater that theupper threshold, then there is no blockage; then the activity issufficient to demonstrate that air is passing through the inflation line12. In one embodiment, the value of the upper threshold is 8.Accordingly, if step 835 determines the activity is greater than theupper threshold, step 840 continues by reloading the timer.

On the other hand, if step 825 determines that the timer has notexpired, step 850 resets the blockage alarm flag and step 855 determineswhether the current activity is greater than a lower threshold. In oneembodiment, the lower threshold is 3. If step 855 determines that theactivity value is above the lower threshold, then the activity is belownormal activity levels indicated by upper threshold and step 860continues by reloading the Blockage timer.

The described device and its method of use acts as a controller of thecuff pressure of an airway device such as an endotracheal tube. A deviceincorporating aspects of the present invention will perform one or moreof the following functions:

a) perform pressure corrections, at a controlled rate, in response toperiodically measured pressure changes that do not arise from normalrespiratory movement so as to maintain the pressure within a desirablerange (preventing overshoot and undershoot);

b) monitor for leaks in the pressure system and raise an alarm if a leakis detected;

c) monitor for deviations from the correct position of the airway devicethat results in partial or full extubation and raise an alarm if amalposition is detected;

d) monitor for blockage in the pressure system and raise an alarm is ablockage is detected.

These functions may be included in combination with existing functionsof prior art devices, such as that disclosed in WO-A-99/33508.

The device is controlled by a microprocessor 50 which performs all tasksincluding, but not limited to, diagnostic checks, motor and valveoperation and control, and pressure measurement. Typically, the deviceincludes a user interface 52 including a graphic display for pressureindications and alarms, and control unit with an interface providing foroutputting data for advanced monitoring and control thus permitting dataevaluation with different commercial software packages.

In its normal regulation mode, system pressure sampling is at every 0.5seconds, which provides sufficient time between samples to performevaluation against set-point pressure, and issue control signals, in theform of a train of pulses, to the motor controller to drive the pistonto displace air in the closed volume system, and thus change thepressure to substantially setpoint pressure.

A system incorporating several or all the aspects of the presentinvention, discussed above, provides considerable assistance to theanaesthetist during a surgical procedure on a patient. In particular,such a system provides several concurrently operative algorithms toprocess, in real time or in close to real time, the operating status ofthe airway device, and audibly and/or visually alerts the anaesthetistof any adverse changes.

As the skilled person will appreciate, various changes and modificationscan be made to the described embodiments. It is intended to include allsuch variations, modifications and equivalents which fall within thescope of the invention, as defined in the accompanying claims.

1. A method of detecting a leak of fluid from an inflatable airwaydevice, the method comprising: receiving a pressure value for theinflatable airway device; calculating a statistical average of a valuescalculated using the received pressure value and a predetermined numberof previously received pressure values; and if the statistical averageexceeds a threshold average, indicating a leak of fluid from the airwaydevice.
 2. A method as claimed in claim 1, wherein the values calculatedusing the received and previously received pressure values are valuesrepresenting a rate of change in volume or pressure of fluid within theinflatable airway device.
 3. A method for detecting a leak of fluid froman inflatable airway device, the method comprising: periodicallyreceiving a pressure value for the inflatable airway device; comparingthe received pressure value with a predetermined, minimum pressurevalue, and if the received pressure value is less than the predeterminedminimum pressure value, indicating a leak of fluid from the airwaydevice.
 4. A method as claimed in claim 3, wherein the predefinedminimum pressure value is a predetermined proportion of a preselected,desired pressure value, for example 50% of the preselected pressurevalue.
 5. A method for detecting a blockage in a pressure system of aninflatable airway device, the method comprising: receiving a pressurevalue for the inflatable airway device and calculating a currentactivity, and, if the current activity is below a threshold for activityfor a predetermined time period, indicating a blockage in the pressuresystem of the airway device.
 6. A method for detecting a blockage in apressure system of an inflatable airway device, the method comprising:comparing a received pressure value for the inflatable airway device,with a predetermined or average pressure value to calculate adifference; calculating a statistical average of the difference and apredetermined number of differences calculated using previously receivedpressure values; comparing the statistical average with a thresholdaverage, and, if the statistical average is less than the threshold fora predetermined time period indicating a blockage in the pressure systemof the airway device.